5D, 6D vs 7D; what do you get for the extra money?

I've got a 7D and I use it for mostly outdoor nature photography.  I take a lot of pictures but I don't make money from them so the price for even the 6D is a bit of a barrier.  I was wondering if you folks who have 5Ds and 6Ds could comment on what added benefit those cameras could provide for nature photography.
The better low light capability would obviously be good but how much better is it than the 7D at ISO 800?
Also I've got Sigma lenses , I use the Sigma 50-500 most of the time, and I'm wondering if those less expensive lens get too bad when used with a full frame sensor.
Thanks for any information and I hope everyone  has a healthy, happy, Holy Christmas!